Erin, Donald's suggestion for Efke film is a good place to start. With very flat light, use the 25. With "normal" light I would use the 100 and do as he suggested. The 25 doesn't like under exposure. Shadows will turn to coal lumps pretty fast.
Your professor is most likely trying to get you to understand the relationship between exposure and development. In order to make a negative with more contrast, more development is in order. How much more contrast? If it is a stop, you might try between 20% and 30% more development as a starting point. A fim like Tri-x will run out of steam pretty fast on plus development, so make sure you use a film with the ability to build contrast beyond the norm, or your tests will not be too good. Both Efke products mentioned here will do the job well, or even too well, so you will have to play with exposure and development to get what you need. tim